Corrosion resistant,heat resisting valve steel



United States Patent 3,527,600 CORROSION RESISTANT, HEAT RESISTING VALVE STEEL Masaru Yano, Yutaka Sogami, Tokushige Iwata, and

Nobuyoshi Hojo, Kyoto-shi, Japan, assignors to Mitsubishi Jukogyo Kabushiki Kaisha, Tokyo, Japan No Drawing. Filed Dec. 6, 1967, Ser. No. 688,345 Claims priority, application Japan, Dec. 10, 1966, ll/80,890 Int. Cl. C22c 39/26 US. Cl. 75-125 4 Claims ABSTRACT OF THE DISCLOSURE Corrosion resistant, heat resistant steel, particularly for making valves, which essentially consists of the following components: C=0.410.64%, Si=0.05l.2%, Mn=5.5 6.9%, Ni=0.7l.9%, Cr=23%, Cu=0.35l.35%, N: 0.3-0.5 and the rest comprising iron and impurities.

The present invention relates to heat resistant austenitic steel which is of excellent corrosion resistance and which can be manufactured at low costs. It can be used for making heat resisting parts, such as valves of an engine working under heavy loads in an atmosphere of combustion gas products including CO Austenitic steel which is widely used as material for engine valves, comprising a system of Cr-Mn including nitrogen or a system of Cr-Mn-Ni, generally exhibits great strength at high temperatures, and resistance to corrosion by lead compounds. But its corrosion resistance to that atmosphere including sulfur, which is corrosive at high temperatures, is not satisfactory. For example, when an exhaust valve of a diesel engine consuming lower gradefuels is made of austenitic steel, it is prematurely subjected to pitting. As a result, its useful life is considerably shortened.

The present invention is based on the study of resistance to corrosion produced by 80;, that is contained in combustion gases comprising sulfur. It is an object of the present invention to provide heat resisting austenitic steel containing Ni and Cu which are present in small amounts.

It is another object of the invention to provide austenitic steel of marked resistance to corrosion by sulfur contained in the waste gases of combustion engines.

It is yet another object of the invention to provide austenitic steel for the manufacture of valves and the like parts which when exposed to combustion gases will have a longer useful life and can be made at low expense.

Other objects and advantages of the invention will become apparent from the following detailed description.

The heat resisting austenitic steel of the present invention has the following composition: C=0.4l-0.64%, Si=0.05l.2%, Mn=5.5-6.9%, Ni:0.71.9%, Cr=20 23%, Cu=0.351.35%, N=0.30.5%, and the rest comprising iron and impurities.

The function of each component of the steel of the present invention and the reasons, why the content of said component is limited as indicated above, will be explained below.

Patented Sept. 8, 1970 In order to stabilize the austenitic structure of said steel, C is necessary in combination with Mn, Ni and N. Further, in order to obtain its cold hardness over HrC O to resist wear, the carbon content has to be over 0.4%. However, if the content in C exceeds 0.65%, the ductility of the steel is markedly reduced, while its corrosion resistance and hot-working characteristics are also reduced. Accordingly, the content of C is determined to 0.4l0.64%.

Since Si is added as a deoxidizer, its content has to be at least 0.05%. Even with a content over 1.2% the effect of increasing the oxidation resistance of the steel is not substantial. Accordingly, the content in Si is fixed at 0.05 1.2%. It is preferable to obtain the range of 0.05-0.35% for increasing resistance of the steel to corrosion by lead oxides.

The content in Mn has to be at least 5.5% or more, in order to stabilize the austenitic structure together with Ni and N. However, the increase in the high temperature strength of the steel as well as in its corrosion resistance, cannot be expected even with Mn over 7%. Accordingly, its content in the steel is limited to 5 .56.9%.

Ni is a requisite component together with Mn and N in order to stabilize the austenitic structure, and therefore has to be at least 0.5%. Because of its high cost and because of the use of Cu as a corrosion resisting agent, an upper limit of the content of Ni is fixed at 1.9%.

The Cr content has to be over 20% in order to obtain suflicient resistance to corrosion and to heating. On the other hand, in order to stabilize the austenitic structure and in order not to reduce malleability, the upper limit of Cr is fixed at 23%.

Cu of at least 0.35% is necessary for the resistance to the corrosion by S0 But in order to keep its characteristics in hot-working, its upper limit is fixed at 1.35%.

N has to be present in an amount of at least 0.30%, in order to stabilize the austenitic structure, and in order to increase high temperature strength because of precipitation hardening after aging and because of the addition of Cu. However, if the content of N is over 0.5%, then the drawing finish and the machinability of the steel is reduced. Accordingly, said content of N is fixed at 0.30.5%.

Furthermore, as regards the impurities, P is generally below 0.035%; S is generally below 0.10% to obtain better machinability. Such contents of P and S will not prove any obstacle to the effects of the present invention.

Detailed explanations of embodiments of the present invention will, by way of example, be given below.

In Table I, compositions of various steels are indicated in the order of increasing Ni content (except for No. 1). Those steels include known ones tested for comparison, as well as several steels of the present invention. No. 1 found in the table represents G192, a steel to be found in the market; Nos. 5 and 8 represent EVll and EV8 of SAE, Standard, respectively. All samples are made of age hardening austenitic steel. The heat treatment therefor was performed in one and the same manner; after one hour at 1,150 C., water-cooling for solution heat treatment; thereafter ten hours at 750 C. for aging. Corrosion test pieces, 9 mm. in diameter and 10 mm. in length were used.

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TABLE I.COMPOSITIONS OF SAMPLES (PERCENT) Alloy No. C Si M11 P S Cr N N 1 Cu Remarks 0. 59 0.80 8. 90 0.014 0.007 21. 30 0.39 0.20 0.04 G 192. 0. 64 0.48 7. 62 0. 025 0. 008 20. 51 0. 37 0. 04 1. 12 Reference sample. 0. 46 0. 28 6. 88 0. 014 0. 009 20.34 0.45 0.84 0.39 The present invention. 0. 55 0. 17 b. 54 0. 012 0. 008 20. 83 0. 43 1. 35 0. 08 Reference sample. 0.68 0.80 6.87 0. 014 0 005 20. 60 0. 28 1. 41 0.04 EVll. 0. 57 0. 22 5. 88 0. 028 0. 008 21. 67 0. 44 1. 48 0. 64 The present invention. 0. 47 0. 12 6. 76 0. 007 0. 002 20. 11 0.41 1. 1. 00 Do. 0. 55 0.33 9. 20 0. 018 0. 003 20. 10 0.44 3.80 0.08 EVS. 0. 57 0.23 9. 51 O. 027 0. 008 20. 22 0. 40 3. 93 0. 71 Reference sample. 0. 52 0. 28 8. 50 0. 016 0. 005 21. 33 0.38 5. 88 0. 07 Do. 0. 43 0.35 9. 05 0. 016 0. 010 20. 11 0. 27 7. 07 0. 05 D o. 0.31 0. 16 8. 59 0. 032 0. 006 18. 0.33 10. 45 0. 07 Do.

In Table II, the results of S corrosion tests using samples shown in Table I are indicated. The efiect obtained by adding Cu of the present invention is clearly to be seen. In other words, by adding Cu of about 0.7%, any weight loss on 50;, gas corrosion is below 50% in the range of lower contents of Ni. It will thus be noted that steels of the present invention are superior, in corrosion resistance, to high nickel steels.

TABLE II.SO3 CORROSION TEST RESULTS (75 C, 55 HRS.)

Composition, percent Weight loss on corrosion, Ni Cu (10-- n1g./cn1.

TABLE III.HIGH TEMPERATURE STRENGTH RESULTS (750 0., 100 hrs.)

Creep rupture Bending fatigue Alloy strength, kg./mn1. strength, kg./mm.

No. 1 (G192) 12.0 16.0 No. 5 (EVll) 12.5 17.8 No. 8 (EVS) 14. 3 20. 0 No. 6 (the present invention) 14. 2 20. 6

As described above, corrosion resistant and heat resistant valve steel is provided in accordance with the present invention, which spells a remarkable improvement over Cr-Mn heat resistant steels including N. Furthermore, the steel of the present invention has greater strength at high temperatures, and is not expensive to manufacture because the content of Ni is limited. Accordingly, the steel is a very important addition of materials for use by industries making valves for combustion engines and similar parts.

What is claimed is:

1. Corrosion resistant, heat resisting steel, particularly for making valves which essentially consists of 0:041- 0.64%, Si=0.05-1.2%,- Mn=5.56.9%, Ni=0.7-1.9%, Cr=2023%, Cu=0.35-1.35%, N=0.30.5% and the rest comprising iron and impurities.

2. The steel according to claim 1, which consists of C=0.46%, Si=0.28%, Mn=6.88%, P=0.014% S=0.009%, Cr=20.34%, N=0.45%, Ni=0.84%, Cu: 0.39% and the rest comprising iron.

3. The steel according to claim 1, which consists of C=0.57%, Si=0.22%, Mn-=S.88%, P=0.028%, S=0.008%, Cr=21.67%, N=0.44%, Ni=1.48%, Cu: 0.64% and the rest comprising iron.

4. The steel according to claim 1, which consists of C=0.47%, Si=0.12%, Mn=6.76%, P=0.007%, S=0.002%, Cr=20.11%, N=0.4l%, Ni=1.70%, Cu: 1.00% and the rest comprising iron.

References Cited UNITED STATES PATENTS 3,165,400 1/1965 Roy 126 3,310,396 3/1967 Hochmann. 3,401,036 9/1968 Dulis 7s 12s.s

HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75-128 

